I am pretty sure it was on GTs prior to 13. It is called a skip shift. This increases the fuel milage so u r not charged a tax for high hp/ low fuel econ cars. Under normal driving, it will lock out a shift from first into second, and only allow a 1-4 shift. A real pain in the ***. Under wot, it works fine 1-2. U can either tune it out, or buy a skip shift eliminator that. Plugs into the tranny wire.
This was how it was on my 2006 GTO anyway, and pretty sure the same for the Stangs. Btw, my '14 GT does not have this.